单项选择题

Questions 21-26
Complete the summary below.
Choose your answers from the box and write the letters A-N next to questions 21-26.
A wetland is an area where the soil is typically water-logged. Plants and animals living there depend on the we ness for their (21) Draining swamps is a widespread occurrence which kills off wildlife and, consequently, wetlands are (22) worldwide. It is a feature of wetlands that conditions vary according to (23) Water-tolerant plants grow both in and out of the water and water levels are usually (24) Wetlands naturally occur between land and water and become (25) for various wildlife during very dry periods. They also act as nurseries for different kinds of animal life. Wetlands are known to upgrade (26) by removing pollutants.

21（）

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